This invention relates to an automatic speed control circuit for an electric motor, and especially to a circuit for controlling the voltage between two terminals of the motor.
In the automatic speed control circuit of the above mentioned type, it is typical to detect the speed of a motor represented by revolutions per minute (r.p.m.) and convert the detected input into a d.c. level corresponding to the speed of the motor, and thereafter, the d.c. level is compared with a reference level of a fixed value to develop a difference or error signal supplied to a d.c. amplifier circuit driving the motor.
The d.c. amplifier circuit to which a stabilized voltage is given from a voltage stabilizing circuit is provided with an output transistor connected in series with the motor. The serial circuit of the output transistor and the motor is directly coupled across the terminals of a d.c. power source in order to avoid any level reduction of a source voltage due to the voltage stabilizing circuit. Accordingly, non-stabilizing voltage is supplied from the power source to the serial circuit of the output transistor and the motor.
As a result, the output transistor should be connected to the motor so that any change or fluctuation of the source voltage may be removed by the output transistor.
In a conventional control circuit, a pnp transistor is used as the output transistor of the d.c. amplifier circuit. In this case, the emitter of the pnp transistor is coupled to the positive terminal of the d.c. power source while its collector is connected to one terminal of the motor. Further, another terminal of the motor is joined to the negative terminal of the power source which is typically at earth potential. Moreover, a negative feedback circuit comprising a capacitor is inserted between an output side and one input of the d.c. amplifier circuit to reduce the ripple component of a control signal driving the motor.
According to this circuit using a pnp transistor, no fluctuation of the power source affects electromotive force of the motor to be absorbed by the pnp transistor, and any ripple component is also effectively suppressed by the negative feedback circuit.
However, the conventional circuit is unsuitable for a semiconductor integrated circuit because a high power pnp transistor should be prepared by the semiconductor integrated circuit.
On the other hand, when an npn transistor which is easily prepared by the semiconductor integrated circuit is employed as the output transistor of the d.c. amplifier circuit, the npn transistor is inevitably coupled between the negative terminal of the power source and one terminal of the motor to carry out the same control as the pnp transistor. Accordingly, another terminal of the motor should be connected with the positive terminal of the power source which supplies any fluctuating voltage. Thus, in this circuit, since no fluctuating voltage of the power source is absorbed by the npn transistor, a voltage between both terminals of the motor irregularly varies by any fluctuation of the power source and the resultant hunting occurs in the motor.